1. Field of Invention
The present invention relates to the general field of probes intended for ultrasonic imaging, also called “echographic imaging”.
The invention more particularly relates to methods and probes with which viscoelastic properties may be imaged, based on the use of ultrasonic radiation pressure.
2. Description of Related Art
Conventional echographic probes are designed for both transmitting ultrasonic waves into the tissues in a medium and sensing back-diffused signals in order to analyze them and to form an image of the medium.
Typically, these probes consist of a series of N piezoelectric transducers aligned along a line. This line may be straight or curved.
Piezoelectric transducers are individually controlled via electronic routes capable of applying electric signals out of phase with each other.
By adjusting the phases and/or delays according a cylindrical law, it is possible to focus an ultrasonic beam onto a given location in the medium, thereby electronically creating the equivalent of a lens. These laws are also used in the receiving steps in order to isolate back-diffused signals from a given location of the medium and reconstruct its acoustic image.
The size and the spacing of the transducers depend on the frequency of the ultrasonic probe and typically vary between 0.5 and 1 wavelength of the emitted ultrasonic waves.
With such a one-dimensional wave, electronic focussing and reconstruction of the ultrasonic image can only be achieved in a plane.
In the third dimension, called the “elevation”, it is customary to apply on the piezoelectric transducers a fixed geometrical lens with which the ultrasonic beam may be confined on a section of reasonable thickness.
Thus, typically, the elevational size of the piezoelectric transducers is 20 wavelengths of the emitted ultrasonic wave and the geometrical focussing depth of 100 wavelengths of the emitted ultrasonic wave.
Ultrasonic waves are sometimes used for creating transient changes in the medium, for example pressure of ultrasonic radiation.
The use of ultrasonic radiation pressure is used in elastographic techniques. These techniques are imaging modes, additional to those of standard echographic imaging.
However, the use of standard echographic linear arrays particularly designed for providing a very high quality echographic image is not optimum for applying elastographic techniques and more generally for producing transient modifications within the medium.
The geometrical and acoustic properties of known probes are not suitable for generating internal mechanical stresses.
Further, the quality of the resulting elastographic images is not satisfactory.
In the case of elastographic techniques, limitations induced by known probes are three in number.
First of all, the penetration depth of the mechanical stress is limited, generally to half the potentially explorable depth.
Next, the width of the exploration area is also limited because the internal mechanical vibration source has an unsuitable geometry.
Finally, very intense acoustic fields are created so that the internal mechanical stress may be generated.
The intensity of these acoustic fields may exceed the current exposure limits and be dangerous for the patients.